To evaluate the influence of physical training on myocardial blood flow arriving via coronary collateral channels, we used radioactive microspheres to measure perfusion of the myocardium at rest and during exercise before and after physical training in dogs with multiple coronary occlusive lesions. The endocardial portion of the region of myocardium perfused occlusively by coronary collateral channnels showed evidence of deficient flow, especially during exercise. Unlike other portions of the heart and in contrast to the myocardium of non-exercised control animals, these underperfused endocardial regions showed a substantial increase (40 percent rise) in blood flow during exercise after a 6-week program of treadmill exercise, suggesting an associated, training-induced improvement in collateral function.